Changing mounting location with front motorplate is different because you have solid surfaces you are mounting to (ie water pump mounts) that are designed to hold weight vs if you weld to side of a block you could be going to coolant passages where section are weaker. If anything you could machine off the mounting locations and then weld to the stock locations as they were designed for the weight and stresses.

You can weld to cast iron its just you are heat treating the block and the areas you are welding to are not designed for certain forces/loads.

For an old truck you would make plates to adapt to the stock mounting locations on the block or you would modify the truck side of things to mount to. Just like any brackets like schumacher or the new holley brackets for gen 3 hemi are just adapted to fit the block locations and then connect to stock engine mounting locations on the older cars.

Like said previously it could be done but how long will it last and what will break first who knows? If you had a repaired block with missing mounts and had a beater to build then yeah could be worth trying.